Hydraulic tilting device for tilting a vehicle cab

ABSTRACT

A hydraulic tilting device for tilting a cab of a vehicle between a driving position and a tilted position, which cab is resiliently supported on the chassis of the vehicle.  
     The tilting device comprises a reservoir for hydraulic fluid and a pump connected to the reservoir. The tilting device further comprises a double-acting hydraulic tilting cylinder for tilting the cab.  
     Near the tilting cylinder a lost-motion conduit is further provided, extending between ports opening into the cylinder space of the tilting cylinder, which ports are in communication with the push chamber and the pull chamber respectively of the tilting cylinder if the piston/piston rod assembly is in the lost-motion range defined by the ports.  
     The tilting device is designed in such a way that—under the influence of the movements of the piston/piston rod assembly up and down within the lost-motion range—by way of the pull connection of the tilting cylinder a suction of hydraulic fluid out of the reservoir occurs, so that hydraulic fluid passes from the reservoir into the pull chamber, and that by way of the lost-motion conduit hydraulic fluid is moved from the pull chamber to the reservoir, so that a circulation of hydraulic fluid occurs in its entirety if the cab is carrying out spring movements.

BACKGROUND OF THE INVENTION

[0001] It is generally known from the prior art to connect a cab of avehicle in a tiltable manner to the chassis of the vehicle, so that thecab can be tilted between a driving position, in which the vehicle canbe driven, and a tilted position, in which, for example, maintenance canbe carried out on the engine situated below the cab. It is furthergenerally known for such a tilting cab in the driving position to beresiliently supported on the chassis in order to provide greater comfortfor those in the cab, in particular the driver, while the vehicle isbeing driven.

[0002] For tilting such a resiliently mounted cab, it is known to use ahydraulic tilting device with a tilting cylinder disposed between thechassis and the tilting cab. In order to ensure that while the vehicleis being driven the tilting cylinder does not interfere with the springmovements of the cab relative to the chassis, tilting devices with aso-called lost-motion mode are used. These tilting devices can bedivided largely into two types, on the one hand, a mechanical type with,for example, a lost-motion arm, which is usually pivotably connectedbetween the tilting cylinder and the cab, or with a sort ofpin-and-groove connection between the tilting cylinder and the cab, and,on the other hand, a hydraulic type.

[0003] In the case of a tilting device with a hydraulic lost-motion, thetilting cylinder exhibits a lost-motion effect. A hydraulic cab tiltingdevice of this type is known, for example from GB 2 079 378.

[0004] It is proposed in this publication that when the vehicle is beingdriven and the cab is carrying out spring movements, thepiston/piston-rod assembly can move up and down unimpeded, since thepiston is in a lost-motion range that is defined by the two ports of thelost-motion conduit. In this case the pump of the tilting device is outof action in this lost-motion mode.

[0005] In the case of this known tilting device anti-suction valves areprovided in the fluid line system in such a way that in the lost-motionmode said valves ensure that fluid cannot be sucked out of the reservoirand cannot enter the cylinder space of the tilting cylinder. The resultthat is intended to be achieved with this in the case of the knowntilting device is that the movement up and down of the piston/piston rodassembly in the lost-motion mode should lead to the displacement of acertain quantity of hydraulic fluid out of the tilting cylinder. Theobject of this is to create a vacuum in the cylinder space of thetilting cylinder, since the suction of fluid out of the reservoir isblocked. The object of the vacuum is to impede the movement up and downof the piston/piston rod assembly as little as possible in thelost-motion mode, so that the spring movement of the cab directlyconnected to the piston/piston rod assembly is not disrupted.

[0006] It appears from experiments that the tilting device proposed inGB 2 079 378 does not function as expected. In particular, it appearsthat the functioning of the lost-motion mode in the manner described inGB 2 079 378 cannot be guaranteed during the envisaged service life ofsuch a tilting device. It is pointed out that during that service lifethe piston/piston rod assembly will move up and down many millions oftimes in the lost-motion mode.

[0007] It is in fact found that the known tilting device is susceptibleto two phenomena.

[0008] The first phenomenon is that, taking into account the envisagedservice life, there is a great chance that at a certain point in timethe sealing effect of the piston rod seal will be inadequate, so thatair will be able to enter the cylinder space. In this case the vacuumthat prevails during the lost-motion mode has an adverse effect on thatsealing effect.

[0009] The second phenomenon is that the valve in the lost-motionconduit is set to open at a certain hydraulic pressure and otherwise toseal off that lost-motion conduit. Again taking into account theenvisaged service life, the functioning of this valve will also deviatefrom the envisaged functioning after some time, for example owing toalmost unavoidable wear. In particular, there is a considerable chanceof the valve at some point opening at a pressure that is lower than thatintended.

[0010] As a result of these phenomena, it can happen that air is suckedin along the piston rod seal. Such air, once sucked in, remains in thehydraulic system of the tilting device. The air sucked in causes anexcessively large quantity of fluid to be forced out of the tiltingcylinder to the reservoir, by way of the lost-motion conduit and thepush chamber, with the result that the pressure in said reservoirincreases and the reservoir may overflow. The quantity of fluid forcedout is particularly great if the opening pressure of the valve in thelost-motion conduit is lower than was originally envisaged, which can bethe consequence of the almost unavoidable wear of that valve.

[0011] The air sucked in does not interfere greatly with the lost-motioneffect of the tilting cylinder, but constitutes a problem in particularwhen the cab is subsequently being tilted by means of the tiltingdevice. It can then, in fact, happen that the cab tips forward abruptlywith an enormous bang as soon as the centre of gravity of the cab passesthe pivot point relative to the chassis. The cause of that undesirableand potentially dangerous movement of the cab is the air present in thepull chamber of the tilting cylinder.

[0012] Another risk is that so much air is sucked in that hydraulicfluid flows out of the reservoir and a shortage of fluid arises. Thatshortage can be such that the tilting of the cab cannot be carried outproperly or at all. There is also a considerable chance then of theshortage of hydraulic fluid being made up, but of the tilting device, inparticular the pull chamber, being inadequately vented. The problemmentioned earlier can occur again as a result of this.

OBJECT OF THE INVENTION

[0013] The present invention aims to provide a reliably functioning cabtilting device with a hydraulic lost-motion effect.

SUMMARY OF THE INVENTION

[0014] The present invention achieves this object by providing a tiltingdevice, which is characterized in that the tilting device is designed insuch a way that during the movements up and down of the piston of thepiston/piston rod assembly within the lost-motion range hydraulic fluidis supplied from the reservoir to the pull chamber by way of the pullconnection, and in that hydraulic fluid is moved from the pull chamberto the reservoir by way of the lost-motion conduit, possibly by way ofthe push chamber, so that a circulation of hydraulic fluid occurs in itsentirety if the cab is carrying out spring movements.

[0015] The tilting device according to the invention is based on thefollowing insight. The supply of hydraulic fluid occurring time andagain from the reservoir to the pull chamber is caused by suction fromthe pull chamber and/or propulsion from the reservoir. This nowdesirable supply of fluid ensures that the pressure in the pull chambercan constantly recover. It is pointed out here that the reservoir, whichis an airtight construction, acts as an accumulator, in which fluid isstored first—which is accompanied by an increase in pressure—and fluidis then released again to the pull chamber. In order to achieve theeffect described above, only a slight fluid volume need be circulated ineach case. It is even considered advantageous for the circulated volumeto be as small as possible, because when large fluid volumes arecirculated the flow resistances in the hydraulic system interfere withthe envisaged circulation effect. The correct circulation can beobtained by synchronizing the opening pressures of the valves in thesystem and the resistances formed by the various throttling devices andthe lines.

[0016] Advantageous embodiments of the tilting device according to theinvention will be described below with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 shows the hydraulic diagram of an exemplary embodiment ofthe tilting device according to the invention,

[0018]FIG. 2 shows the hydraulic diagram of another exemplary embodimentof the tilting device according to the invention, and

[0019]FIG. 3 shows the hydraulic diagram of another exemplary embodimentof the tilting device according to the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0020] The hydraulic tilting devices according to the invention shown inFIGS. 1, 2 and 3 are intended for tilting the driver's cab of a vehicle,in particular of a lorry. Such cabs are usually fitted on the chassis ofthe vehicle and are connected to the chassis so that they can tilt byway of pivoting means. The pivoting means are designed in such a waythat the cab can tilt between a driving position, in which the vehiclecan be driven, and a tilted position, generally forward, in which, forexample, maintenance can be carried out on the vehicle, in particular onthe engine of the vehicle, which is fully or partially situated belowthe cab.

[0021] Such vehicles are also usually provided with resilient cabsupporting means, which support the cab in the driving position, in sucha way that in the driving position the cab can spring up and downrelative to the chassis, thus increasing the comfort of the driver. Thehydraulic tilting device is provided for tilting the cab forward andback again.

[0022] The tilting device comprises an airtight reservoir 1 forhydraulic fluid, the inside of the reservoir being sealed off from theoutside air. A pressure-limiting valve 2 limits the pressure in thereservoir 1 to a predetermined pressure value above atmosphericpressure, for example to a pressure of 1.5 bar above the outside airpressure.

[0023] The tilting device of FIG. 1 further comprises a pump 3, which isconnected to the reservoir 1, and in this case is of the type with asingle delivery port 4 for delivering fluid under pressure, for examplea hand pump.

[0024]FIGS. 1, 2 and 3 further show a double-acting, linear hydraulictilting cylinder 5, for tilting the driver's cab (not shown). Thecylinder 5 comprises a cylinder housing 6, containing a cylinder space7, in which a piston/piston rod assembly can move up and down. Thepiston/piston rod assembly comprises a piston 8 and a piston rod 9immovably connected to it. Around the piston 8 lies a sealing ring 10,while the piston rod 9 projects outwards out of the cylinder housing 6through a piston rod seal 11. The cylinder 5 is normally disposedbetween the cab and the chassis of the vehicle, the cylinder housing 6generally being connected in a swivellable manner to the chassis, andthe end of the piston rod 9 generally being connected in a swivellablemanner to the cab.

[0025] The piston/piston rod assembly 8, 9 forms a pull chamber 12 inthe cylinder space 7, which pull chamber retracts the piston/piston rodassembly when hydraulic fluid is supplied to it, and a push chamber 13,which extends the piston/piston rod assembly when hydraulic fluid issupplied to it. A corresponding pull connection 14 and push connection15 are provided for the pull chamber 12 and the push chamber 13respectively.

[0026] A first hydraulic line 16 connects to the pull connection 14, anda second hydraulic line 17 connects to the push connection 15. The lines16, 17 are connected at the other end to a corresponding port 18, 19 ofa two-position control valve 20. Said control valve has two furtherports 21, 22, which are in communication with the delivery port 4 of thepump 3 and with the reservoir 1 respectively.

[0027] A non-return valve 23, which closes in the direction of the pump3, is accommodated in the connection with the delivery port 4.

[0028] In the second line 17 a hydraulically operated non-return valve30 is accommodated near the push connection 15, which non-return valvecloses in the direction of the valve 20 and opens if sufficienthydraulic pressure is present in the control line 31 connected to theline 16.

[0029] A suction-blocking device is accommodated in the second line 17,between the non-return valve 30 and the control valve 20, whichsuction-blocking device prevents unimpeded suction of fluid through theline 17. That device could be composed of two non-return valves 34, 35positioned parallel to each other and working in opposite directions, asshown diagrammatically in the figures. However, in practice, provisioncan be made for an O-ring to be fitted in a bore, and for a ball whichat a certain pressure is forced through the O-ring. When the ball hasthen gone past the O-ring, the passage is clear. The rigidity of theO-ring in combination with the dimensions of the O-ring and the ball arelargely the determining factors for the pressure at which the ballpasses through the O-ring, in practice, for example 7 bar at roomtemperature.

[0030] In this embodiment no further valves in addition to the valves 20and 23 (integral with the pump) are present between the pull chamber 12and the pump 3. A throttling device 36 is provided near the pullconnection 14 in the line 16.

[0031] A pressure-relief valve 37, placed between the first line 16 andthe reservoir 1, is provided in order to prevent an excessively highpressure of the fluid in the hydraulic circuit.

[0032] The tilting device also comprises a lost-motion conduit 40 withtwo ports opening into the cylinder space 7, indicated by 41 and 42respectively, it being possible for the port 41 to coincide with thepush connection 15 of the cylinder 5.

[0033] The lost-motion conduit 40 is shown diagrammatically and can bedesigned in various ways. For instance, it is possible for thelost-motion conduit 40 to be formed by a lost-motion line lying alongthe outside of the cylinder housing 6. However, it is also conceivablefor the lost-motion conduit to be provided in the cylinder housing 6. Inyet another variant the lost-motion conduit 40 is provided in a tubeextending upwards from the bottom of the cylinder housing 6 in thecylinder space.

[0034] The lost-motion conduit 40 connects the pull chamber 12 to thepush chamber 13 when the piston 8 is situated between the two ports 41,42 of the conduit 40, which is the case in a position of thepiston/piston rod assembly associated with the driving position of thecab. When the vehicle is being driven the pump 3 is out of action andthe tilting device is operating in its lost-motion mode.

[0035] A non-return valve 43 is accommodated in the lost-motion conduit40, which non-return valve closes in the direction of the port 42, whichport 42 in the lost-motion mode of the tilting device opens into thepull chamber 12.

[0036] The hydraulic tilting device illustrated further comprises ahydraulically operated slide valve 50 of the two-way/two-position 2/2type with spring return. The valve 50 has a first valve port 52, whichis connected to the push chamber 13 of the cylinder 5, and a secondvalve port 53, which is connected to the line 17 between the valve set34, 35 and the non-return valve 30.

[0037] The valve 50 has a sliding element which is slidable in the valvehousing and is hydraulically adjustable to the open position under theinfluence of hydraulic pressure by way of the control line 54 and thecontrol line 55. The control line 54 is directly connected to the pushchamber 13, and the control line 55 is connected to the line 17 betweenthe valve set 34, 35 and the non-return valve 30.

[0038] By way of control line 56, which is in direct communication withthe port 42, a hydraulic pressure can be exerted upon the slidingelement, forcing the sliding element to the closed position.

[0039] If the process is stopped during forward tilting of the cab, andthe cab is going to fall back to the driving position again under itsown weight, then the non-return valve 30 closes. If the piston 8 issituated above the port 42 at the time of stopping, the valve 50 alsoremains closed, because the hydraulic pressure in the push chamber 12then acts upon the sliding element by way of the control line 56. Inthis way the piston/piston rod assembly is held reliably in place, whichis of great importance for the safety of the persons who at the time aremoving to or are already in a position below the cab.

[0040] When the cab has to be tilted out of the tilted position to thedriving position, i.e. the piston rod 9 has to be retracted, fluid issupplied by way of delivery port 4 to the pull chamber 12. The pressurein the first line 16 causes the non-return valve 30 to open, and fluidcan flow out of the push chamber 13. If the tilting back is interruptedin a position in which the piston 8 is situated above the port 42, thevalve 50 is held in the closed position. The valve 30 also closes, sothat the cab then remains at a standstill. If during the tilting back ofthe cab to the driving position the piston 8 has already passed the port42, then the pressure falls away in the part of the lost-motion conduit40 between the valve 43 and port 42. This causes the pressure in theline 56 also to fall away, and the valve 50 is opened through thepressure prevailing in line 54. The result is that the fluid can flowout of the push chamber 13 to the reservoir 1, and the cab can tip backfurther unimpeded to the driving position, an orifice also beingprovided in order to limit the speed of movement of the cab. This“free-fall movement” of the cab is desirable in many cases, in order tomake the cab lock with one or more cab locking devices, which are fittedon the chassis and secure the cab in the driving position.

[0041] A relatively rapid tilting movement of the cab is oftenadvantageous for producing the engagement between the cab and suchlocking devices.

[0042] When the cab is in the driving position and the tilting device isbrought into the lost-motion mode, which here simply means that the pump3 is switched off and the valve 20 is in its illustrated position, thenthe device shown in FIG. 1 works as follows.

[0043] When the cab makes such a spring movement that the piston rod 9is drawn out of the cylinder housing 6, fluid flows out of the pullchamber 12 by way of the port 42, and then passes by way of the line 40,through the opening non-return valve 43, into the push chamber 13. Thevalve 50 remains closed here.

[0044] When the cab moves in such a way that the piston rod 9 is forcedinto the cylinder housing 6, fluid flows out of the push chamber 13 byway of the open valve 50 to the reservoir 1. In this situation the valve50 opens because a hydraulic pressure is exerted upon a control pressureopening surface of the sliding element by way of the control pressureline 54. At the same time no hydraulic pressure is acting upon theclosing surface of the sliding element, so that only the (light) spring58 is counteracting the opening of the valve 50.

[0045] When the cab moves in such a way that the piston rod 9 is forcedinto the cylinder housing 6, hydraulic fluid is also sucked out of thereservoir 1 by way of the line 16. A (slight) vacuum in fact occurs inthe pull chamber 12, which vacuum, possibly in conjunction with thepressure above atmospheric pressure in the reservoir 1, leads to a flowof fluid out of the reservoir, along the weak non-return valve 23 to thepull connection. In this way the pull chamber 12 remains filled withhydraulic fluid. The opening pressure of the valve 23 is preferably low,preferably less than 0.5 bar, for example approximately 0.2 bar. Inorder to prevent too great a flow of hydraulic fluid, the throttlingdevice 36 is provided, which throttling device can be composed of twobores of small diameter for example 0.5 millimetre, placed one behindthe other.

[0046] In its entirety, what is now achieved is that while the vehicleis being driven, during which time the cab constantly springs up anddown, hydraulic fluid is circulated through the hydraulic circuit of thetilting device. The direction of flow here is from the reservoir 1 tothe pull chamber, by way of the lost-motion conduit 40, to the pushchamber 13, and from the push chamber 13, by way of the valve 50, backto the reservoir 1.

[0047] The first advantage of the circulation of hydraulic fluid is thatfluid is supplied constantly to the pull chamber 12, so that even if airwere to leak along the piston rod seal 11 into the pull chamber 12, itwould be a minimum quantity here. The pressure in the pull chamber 12can thus constantly adjust to the value set by the valve 43.

[0048] The pressure in the pull chamber 12 is now determined by theopening pressure of the valve 43 in the lost-motion conduit 40 and thepressure in the push chamber 13. In practice, a maximum pressure of, forexample, 2.5 bar above atmospheric pressure could be achieved in thepull chamber 12 in the lost-motion mode. If that pressure prevails inthe pull chamber 12, in the embodiment illustrated a pressure ofapproximately 1 bar below atmospheric pressure prevails in the pushchamber. The maximum pressure in the push chamber 13 is then between 0.5and 2 bar above atmospheric pressure, which last value prevails at aminimum pressure in the pull chamber 12.

[0049]FIG. 2 shows a tilting device which corresponds largely to thetilting device according to FIG. 1. The difference is in particular thatin this case a preferably electrically driven pump 60 of the reversibletype is used, having two ports 61, 62, which serve as delivery port orsuction port depending on the pumping direction. Components of thetilting device in FIG. 2 which correspond to components shown in FIG. 1are provided with the same reference numerals. A suction exchange valve64 and two pressure-relief valves 65, 66 can also be seen in FIG. 2. Inthe light of the description with reference to FIG. 1, it will be clearto the person skilled in the art that here again a circulation ofhydraulic fluid occurs during the lost-motion mode.

[0050]FIG. 3 shows a variant of the tilting device according to FIG. 1.Corresponding parts are provided with the same reference numerals.

[0051] In the case of the tilting device according to FIG. 3 anadditional valve 67 is provided compared with the tilting deviceaccording to FIG. 1. Said valve 67 is fitted in the common part of thelost-motion conduit 40 and the second line 17, which at the common port41, 15 connects to the push chamber 13 of the cylinder 5. Thelost-motion conduit 40 is thus in communication with the second line 17.

[0052] The valve 67 is designed to connect the push chamber 13 to, or incertain conditions to shut it off from, the lost-motion conduit 40 andthe second line 17.

[0053] The valve 67 is a hydraulically controlled slide valve of thetwo-way/two-position 2/2 type with spring reset to the closed positionby means of spring 68.

[0054] The valve 67 has a sliding element which is hydraulicallyadjustable under the influence of hydraulic pressure by way of thecontrol lines 69, 70, 71.

[0055] When hydraulic fluid is supplied under pressure by means of thepump 3 to the second line 17, the valve 67 opens under the influence ofthe hydraulic pressure by way of control line 71. If the piston 8 iswithin the lost-motion range, the valve 67 will open as soon as theforce by way of the pressure along control line 71 is equal to the forceof the spring 68. If the piston 8 is outside the lost-motion range, thevalve 67 opens if the pressure at control line 71 is equal to thepressure in the push chamber 13 and the force of the spring 68.

[0056] When hydraulic fluid is supplied under pressure by means of thepump 3 to the first line 16, so long as the port 42 is still connectedto the push chamber 13 the valve 67 will remain closed under theinfluence of the pressure by way of the control line 69 and 70. Thefluid then flows through the lost-motion conduit 40 and by way of thenon-return valve 30 out of the push chamber 13. If the piston 8 goeswithin the lost-motion range, the pressure in control line 69 falls awayand the valve 67 is opened by the pressure in control line 70.

[0057] During the inward movement of the piston 8 in the lost-motionmode of the tilting device a build-up of pressure in the push chamber 13causes the valve 67 to be opened by way of the control line 70.

[0058] During an outward movement of the piston 8 in the lost-motionmode a build-up of pressure occurs in the control lines 69 and 71. Owingto the fact that the surfaces upon which these control pressures act aredifferent, the valve 67 remains closed. The circulation flow of fluidout of the pull chamber 12 occurs through the lost-motion conduit 40directly by way of the valve 30 (which opens under the influence ofcontrol pressure through line 31) to the reservoir 1. This flowtherefore does not pass through the push chamber 13 here.

[0059] As a result of the valve 67, the anti-suction valves 34, 35 inFIG. 1 are no longer necessary.

[0060] It will be clear to the person skilled in the art that theinventive idea described can also be achieved in the case of hydraulictilting devices with a hydraulic diagram that differs in one or morepoints. For instance, it is conceivable for the valves 30 and 43 to beintegral in the slide valve 50.

[0061] The inventive idea can also be implemented in the tilting devicedescribed in DE 197 30 499 with reference to FIG. 1 of that Germanpublication. In this case it is sufficient in the case of that knowntilting device to dispense with the anti-suction valves 29, 30 (assembly132) in the line 16 to the pull chamber of the tilting cylinder and tomake the reservoir an airtight reservoir with excess-pressure valve, asexplained here earlier with reference to FIG. 1 of this presentinvention. No anti-suction valves need be present in the line 20 of theknown tilting device either.

What is claimed is:
 1. A hydraulic tilting device for tilting a cab of avehicle between a driving position and a tilted position, which cab inthe driving position is resiliently supported on a chassis of thevehicle, which tilting device comprises: a reservoir for hydraulicfluid, a pump connected to the reservoir and having a delivery port fordelivering hydraulic fluid under pressure; a double-acting, hydraulictilting cylinder, for tilting the cab, comprising a cylinder housing,containing a cylinder space, and comprising a piston/piston rod assemblyincluding a piston and a piston rod, which piston/piston rod assembly ismoveable back and forth in said cylinder space, the piston rod of whichassembly projects outwards out of the cylinder housing through acorresponding piston rod seal, which piston/piston rod assembly forms apull chamber in the cylinder space, so that the piston/piston rodassembly is retracted when hydraulic fluid is supplied to said pullchamber, and a push chamber, so that the piston/piston rod assembly isextended when hydraulic fluid is supplied to said push chamber, acorresponding pull connection and push connection being provided for thepull chamber and the push chamber, in which the tilting cylinder isfurther provided with a lost-motion conduit, extending between portswhich open into the cylinder space of the tilting cylinder, which portsare in communication with the push chamber and the pull chamberrespectively if the piston of the piston/piston rod assembly is in alost-motion range defined by the ports, the piston/piston rod assemblybeing able to move within said lost-motion range if the cab is carryingout spring movements in its driving position, a valve, which is disposedin the lost-motion conduit and can shut off the lost-motion conduit, aline system, such that the pull connection and the push connection areconnectable to the delivery port of the pump or the reservoir, whereinthe tilting device is designed such that during movements of the pistonof the piston/piston rod assembly within the lost-motion range hydraulicfluid is supplied from the reservoir to the pull chamber by way of thepull connection, and wherein hydraulic fluid is moved from the pullchamber to the reservoir by way of the lost-motion conduit, so that ingeneral a circulation of hydraulic fluid occurs if the cab is carryingout spring movements.
 2. A tilting device according to claim 1, whereinsaid valve is a valve which closes in the direction of the pull chamberand opens at a predetermined opening pressure in the pull chamber.
 3. Atilting device according to claim 1, in which the tilting device isdesigned such that during movements of the piston of the piston/pistonrod assembly within the lost-motion range hydraulic fluid is suppliedfrom the reservoir to the pull chamber by way of the pull connection,and in which hydraulic fluid is moved from the pull chamber to the pushchamber by way of the lost-motion conduit, and in which hydraulic fluidis discharged from the push chamber to the reservoir, so that acirculation of hydraulic fluid occurs in its entirety if the cab iscarrying out spring movements.
 4. A tilting device according to claim 1,in which the reservoir is of the airtight type with a pressure-reliefvalve, in such a way that the maximum reservoir pressure in thereservoir is limited to a pressure above atmospheric pressure, so thatthe reservoir pressure contributes to the supply of hydraulic fluid tothe pull chamber.
 5. A tilting device according to claim 4, in which themaximum reservoir pressure is 1.5 bar above the outside air pressure. 6.A tilting device according to claim 1, in which in the line systembetween the reservoir and the pull connection, in particular between thedelivery port of the pump and the pull connection, if suction from thereservoir is possible by way of the pump, a non-return valve closing inthe direction of the reservoir is accommodated, which non-return valveopens at an opening pressure, which opening pressure is advantageouslyless than 0.5 bar, preferably approximately 0.2 bar.
 7. A tilting deviceaccording to claim 1, in which in the line system between the reservoirand the pull connection a throttling device is accommodated.
 8. Atilting, device according to claim 1, in which in the line systembetween the reservoir and the pull connection a throttling device isaccommodated having one or more bores disposed one after the other andhaving a diameter of approximately 0.5 millimetre.
 9. A tilting deviceaccording to claim 1, in which a hydraulically operated slide valve isprovided, which slide valve has a first valve port, which is connectedto the push chamber, a second valve port, which at least in thelost-motion mode is connected to the reservoir, and a sliding element,which is slidable between a closing position, in which the first andsecond valve ports are shut off from each other, and an openingposition, in which the first and second valve ports are in communicationwith each other, the slide valve being provided with a spring meanswhich loads the sliding element in the direction of its closingposition, and in which an opening control surface is associated with thesliding element, which opening control surface by way of a control lineis in communication with the push chamber, so that a hydraulic pressurein the push chamber acts upon the opening control surface in order toforce the sliding element to the opening position, and a closing controlsurface being present, which by way of a control line is connected tothe cylinder space, so that a hydraulic pressure in the push chamberacts upon the closing control surface in order to force the slidingelement to the closing position.
 10. A vehicle, comprising a tiltingcab, a chassis and pivoting means, which connect the cab to the chassis,in which the cab is tiltable through an angle relative to the chassisabout a pivot pin defined by the pivoting means, between a drivingposition and a tilted position, and in which the cab is resilientlysupported on the chassis, and a hydraulic tilting device according toone claim 1 is provided, in order to tip the cab, the tilting cylinderbeing fitted directly between the chassis and the cab.